BMW 740i 2008-2015 Drivetrain Bolt Torque Specifications

Using the correct torque specifications ensures that bolts are tightened to the optimal tension. Too little torque can cause bolts to loosen, leading to component failure, excessive wear, and potentially dangerous situations like a driveshaft separating. Too much torque can stretch or break bolts, damage threads, and cause components to bind or deform, also resulting in failure. Consistent torque application also contributes to even clamping force distribution.

Over-torquing can lead to several problems. The bolt itself can be stretched beyond its yield point, weakening it and potentially causing it to snap. Threads within the component being bolted can be stripped or damaged. In the case of the driveshaft, over-tightening could damage a rubber coupling or the driveshaft center support bearing housing, leading to premature failure and requiring replacement. Additionally, it could also result in uneven clamping or cause internal component damage. For example, overtightening the bolts on the transmission output flange can damage the oil seal.

Under-torquing allows the bolt to loosen under vibration and load, which can lead to component misalignment, excessive wear on mating surfaces, and potential shearing of the bolt itself. This is especially critical on components like driveshaft bolts, axle flange bolts, and transmission mount bolts. For example, if the bolts holding the transmission to the subframe are loose, you may experience clunking noises, misaligned gears and vibrations. Under-torquing is just as dangerous as over-torquing as it can lead to severe component separation and failure.

It is highly recommended, and in many cases necessary, to use new bolts, especially stretch bolts (Torque-to-Yield bolts), whenever you disassemble and reassemble drivetrain components. Some bolts are designed to stretch slightly when tightened to the specified torque, ensuring proper clamping force. These bolts should not be reused because they may not have the same load-bearing capacity. Reusing bolts can lead to improper clamping force and component failure.

Yes, a calibrated torque wrench is essential. Using an impact wrench or tightening bolts by hand ('feel') is not accurate and can lead to under or over-torquing. For some bolts, a specific socket or extension may be needed, especially where access is restricted. An angle gauge may also be required for 'torque-to-angle' specifications on some bolts which measure the amount of turning after the target torque has been achieved. Using a torque wrench with the correct range is crucial. For example, smaller bolts might require a lower-range in-lb wrench while larger fasteners may need a higher-range ft-lb wrench. Some bolts also require special triple square drive sockets.